Engine Control Module: Description and Operation
SENSOR RETURN - PCM INPUTThe sensor return circuit provides a low electrical noise ground reference for all of the systems sensors. The sensor return circuit connects to internal ground circuits within the Powertrain Control Module (PCM).
DATA BUS COMMUNICATION RECEIVE- PCM INPUT
The PCM uses the SCI communication bus to preform engine diagnostics and flash operations. The transmission side of the PCM uses the SCI communication bus to flash new software. However, diagnostics is performed via the vehicles J1850 bus for the transmission side of the PCM.
IGNITION SENSE- PCM INPUT
The ignition sense input informs the Powertrain Control Module (PCM) that the ignition switch is in the crank or run position.
PCM GROUND
Ground is provided through multiple pins of the PCM connector. Depending on the vehicle there may be as many as two different ground pins. There are power grounds and sensor grounds.
The power grounds are used to control the ground side relays, solenoids, ignition coil or injectors. The signal ground is used for any input that uses sensor return for ground, and the ground side of any internal processing component.
The PCM case is shielded to prevent RFI and EMI. The PCM case is grounded and must be firmly attached to a good, clean body ground.
Internally all grounds are connected together, however there is noise suppression on the sensor ground. For EMI and RFI protection the housing and cover are also grounded separately from the ground pins.
PCM:
The PCM receives input signals from various switches and sensors that are referred to as PCM Inputs. Based on these inputs, the PCM adjusts various engine, transmission, and vehicle operations through devices that are referred to as PCM Outputs.
NOTE: PCM Inputs:
- Air Conditioning Controls
- Ambient Air temperature Sensor
- ASD Sense
- Baro/Tip (Turbo)
- Battery Voltage
- Battery Temperature Sensor
- Brake Switch
- Camshaft Position Sensor
- Clutch Upstop Switch
- Clutch Interlock
- Crankshaft Position Sensor
- Engine Coolant Temperature Sensor
- Fuel Level Sensor (Bus message)
- Ignition Switch
- Intake Air Temperature Sensor
- J1850
- Knock Sensor (2.0, 2.4L)
- Natural Vacuum Leak Detection (NVLD)
- Manifold Absolute Pressure (MAP) Sensor
- Oil Pressure Switch
- Oxygen Sensors
- Power Steering Pressure Switch
- SCI Receive
- Speed Control Switches
- Throttle Position Sensor
- Transmission Control Relay (Switched B+)
- Transmission Input Shaft Speed Sensor
- Transmission Output Shaft Speed Sensor
- Transmission Pressure Switches (L/R, 2/4, OD)
- Transmission Range Sensor (TRS)
- Transmission Oil Temperature Sensor (Integral to TRS)
- Vehicle Speed Sensor (MTX-equipped models)
NOTE: PCM Outputs:
- Air Conditioning Clutch Relay
- Auto Shutdown (ASD) Relay
- Charging Indicator Lamp (Bus Message)
- SCI Transmit
- Proportional Purge Solenoid
- EGR Solenoid
- Fuel Injectors
- Fuel Pump Relay
- Generator Field
- Idle Air Control Motor (2.0/2.4L)
- Ignition Coils
- J1850
- Malfunction Indicator (Check Engine) Lamp (Bus Message)
- Oxygen Sensors Heater Controls
- Radiator Fan Relays
- Speed Control Solenoids (2.0/2.4L)
- Transmission Control Relay
- Transmission Solenoids (LR/CC, 2/4, OD, and UD)
- Transmission PRNDL Position (to Cluster)
- Transmission Torque Reduction Request (Internal to PCM)
- Transmission Temperature (Internal to PCM and a Bus Message)
- Vehicle Speed (Manual Transmission) Based on inputs it receives, the PCM adjusts fuel injector pulse width, idle speed, ignition spark advance, ignition coil dwell and EVAP canister purge operation. The PCM also determines the appropriate transmission shift schedule and shift points, depending on the present operating conditions and driver demand. The PCM regulates the cooling fan, air conditioning and speed control systems. The PCM changes generator charge rate by adjusting the generator field. The PCM also performs diagnostics.
The PCM adjusts injector pulse width (air-fuel ratio) based on the following inputs.
- Battery voltage
- Coolant temperature
- Exhaust gas content (oxygen sensor)
- Engine speed (crankshaft position sensor)
- Intake air temperature
- Manifold absolute pressure
- Throttle position
The PCM adjusts ignition timing based on the following inputs.
- Coolant temperature
- Engine speed (crankshaft position sensor)
- Knock sensor
- Manifold absolute pressure
- Throttle position
- Transmission gear selection (park/neutral switch)
- Intake air temperature
The PCM also adjusts engine idle speed through the idle air control motor based on the following inputs.
- Air conditioning sense
- Battery voltage
- Battery temperature
- Brake switch
- Coolant temperature
- Engine speed (crankshaft position sensor)
- Engine run time
- Manifold absolute pressure
- Power steering pressure switch
- Throttle position
- Transmission gear selection (park/neutral switch)
- Vehicle distance (speed)
The Auto Shutdown (ASD) and fuel pump relays are located in the Power Distribution Center (PDC).
The camshaft position sensor and crankshaft position sensor signals are sent to the PCM. If the PCM does not receive the signal within approximately 1 second of engine cranking, it deactivates the ASD relay and fuel pump relay. When these relays are deactivated, power is shut off from the fuel injectors, ignition coils, oxygen sensor heating elements and fuel pump.
The PCM contains a voltage converter that changes battery voltage to a regulated 5 volts direct current to power the camshaft position sensor, crankshaft position sensor, manifold absolute pressure sensor, throttle position sensor, A/C pressure switch, A/C pressure transducer, and vehicle speed sensor.
Powertrain Control Module Connectors
The PCM is an engine and transmission controller module all in one, if the vehicle is equipped with an automatic transmission. The PCM uses four wiring harness connectors to receive and send engine and transmission data. To ease assembly, the mating wiring harness connector is color-coded. Each module connector cavity has its own unique color identification stripe located on the outside of each connector cavity.
The PCM module utilizes four wiring harness connectors as described:
- Connector Cavity A is for Power & Ground (Black)
- Connector Cavity B is for Engine Side (Orange)
- Connector Cavity C is for Headlamp & Dash (White)
- Connector Cavity D is for Transmission (Green) If equipped
NOTE: Connector Cavities A, B, C, And D must be connected prior to battery connection and ignition key on to avoid setting erroneous controller fault codes. It is also recommended that cavity A connector is made prior to any other connectors.
TRANSMISSION CONTROL
CLUTCH VOLUME INDEX (CM)
An important function of the PCM is to monitor Transmission Clutch Volume Index (CM). CMs represent the volume of fluid needed to compress a clutch pack.
The PCM monitors gear ratio changes by monitoring the Input and Output Speed Sensors. The Input, or Turbine Speed Sensor sends an electrical signal to the PCM that represents input shaft rpm. The Output Speed Sensor provides the PCM with output shaft speed information.
Example Of CVI Calculation:
By comparing the two inputs, the PCM can determine transaxle gear ratio. This is important to the CM calculation because the PCM determines CVIs by monitoring how long it takes for a gear change to occur.
Gear ratios can be determined by using the DRBIII(R) Scan Tool and reading the Input/Output Speed Sensor values in the "Monitors" display. Gear ratio can be obtained by dividing the Input Speed Sensor value by the Output Speed Sensor value.
For example, if the input shaft is rotating at 1000 rpm and the output shaft is rotating at 500 rpm, then the PCM can determine that the gear ratio is 2:1. In direct drive (3rd gear), the gear ratio changes to 1:1. The gear ratio changes as clutches are applied and released. By monitoring the length of time it takes for the gear ratio to change following a shift request, the PCM can determine the volume of fluid used to apply or release a friction element.
The volume of transmission fluid needed to apply the friction elements are continuously updated for adaptive controls. As friction material wears, the volume of fluid need to apply the element increases.
Certain mechanical problems within the clutch assemblies (broken return springs, out of position snap rings, excessive clutch pack clearance, improper assembly, etc.) can cause inadequate or out-of-range clutch volumes. Also, defective Input/Output Speed
TRANSMISSION SHIFT SCHEDULES
The PCM is programmed to allow it to select a variety of shift schedules. Shift schedule selection is dependent on the following:
- Shift lever position
- Throttle position
- Engine load
- Fluid temperature
- Software calibration level
As driving conditions change, the PCM appropriately adjusts the shift schedule. Refer to the following chart to determine the appropriate operation expected, depending on driving conditions.
Sensors and wiring can cause these conditions. The following chart identifies the appropriate clutch volumes and when they are monitored/updated:
5 VOLT SUPPLY - PCM OUTPUT
The PCM supplies 5 volts to the following sensors:
- A/C pressure transducer
- Ambient Temperature sensor
- Battery temperature
- Camshaft Position Sensor (NGC)
- Crankshaft Position Sensor (NGC)
- Electronic Throttle Control (1.6L)
- Engine coolant temperature sensor
- Inlet Air Temperature Sensor
- Knock sensor
- Linear EGR solenoid (if equipped)
- Manifold absolute pressure sensor
- Oil Pressure Switch
- Pedal Position Sensor (1.6L)
- Throttle position sensor
- Vehicle Speed Sensor